Industrial processes often require measuring the level of liquid or other material in a tank. Many technologies are used for level measurement. With contact level measurement some part of the system, such as a probe, must contact the material being measured. With non-contact level measurement the level is measured without contacting the material to be measured. One example is non-contact ultrasound, which uses high-frequency audio waves to detect level. Another example is use of high-frequency or microwave RF energy. Microwave measurement for level generally uses either pulsed or frequency modulated continuous wave (FMCW) signals to make product level measurements. This method is often referred to as through air radar. Through air radar has the advantage that it is non-contact and relatively insensitive to measurement errors from varying process pressure and temperature. Known radar process control instruments operate at frequency bands of approximately 6 Ghz or 24 Ghz.
While tank radar process control instruments measure product level without contact, in most cases part of the instrument must be mounted on the tank and a microwave antenna must be inserted into the tank in order to function. Problems can arise if the medium in the tank is “hazardous”, i.e. it is subject to ignition and/or explosion. Any equipment installed in such locations must meet strict requirements in order to assure that any device, including tank level measurement devices, cannot ignite the vapors, etc., that may be present in such a tank. One method for achieving safe operation is to include a so-called intrinsic safety (IS) barrier in the system design. The concept of the IS barrier is to guarantee that sufficient amounts of energy cannot be transferred into the tank, in this case via the antenna, to cause an explosion. The IS, or energy-limiting barrier, may consist of zener diodes, current limiting resistors, and fuses so that energy levels at the antenna remain safely below published, known ignition curves for the particular process. IS barriers are traditionally placed in the input connections of a process control instrument. Doing so may cause loss of loop power and supply voltage due to the protective components, and produce ground loop product problems, which are difficult to overcome in multiple unit installations. An optimum location for the IS barrier is at the antenna connection. However, placing an IS barrier at the RF stages of the instrument could pose problems. Circuit design factors such as output impedance matching, return loss, agency compliance, and others are typical concerns. Radiated spectrum compliance, and in some cases radar receiver performance, can often be aided by filtering at the antenna connection.
An additional requirement for industrial measurements such as radar process control instruments is a dielectric withstand test. As a measure of reliability, the power connections are shorted together and a relatively high DC voltage is applied between the shorted loop leads and the instrument case (earth ground). To pass the test, the circuit electronics must be able to withstand this voltage from its circuitry to earth ground. An IS barrier placed at the antenna connection may be called upon to withstand this voltage.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.